1. Technical Field
This invention relates to an inverter used, for example, as a driver for fluorescent lamp or EL-board as well as for ionizer or ozonizer and more particularly to an inverter provided with output regulating mechanism suitable for cost reduction.
2. Background Art
FIG. 8 of the accompanying drawings is a circuit diagram illustrating a conventional embodiment of the inverter constructed as the driver for a fluorescent lamp, in which a booster transformer 11 includes an input coil 11P having a center tap P, an output coil 11S and a feedback coil 11f.
A capacitor 12 for resonance is connected in parallel to the input coil 11P and an AC voltage is output from the output coil 11S.
Transistors 13, 14 are switching transistors adapted to be repeatedly turned ON and OFF so that the transistor 13 serves to interrupt the current flowing through a section of the input coil 11P on one side with respect to the center tap P and the transistor 14 serves to interrupt the current flowing through a section of the input coil 11P on the other side with respect to said center tap P.
These transistors 13, 14 are alternately turned ON with the starting current applied to their bases from a source battery 15 through a starting rheostat 16 and operation of these transistors 13, 14 is accelerated under a feedback effect provided by a feedback coil 11f connected to the bases of the respective transistors 13, 14.
The center tap P of said booster transformer 11 is supplied with the source current through the switching transistor 17 and a choke coil 18. During this current supply, said switching transistor 17 is turned ON and OFF in response to the pulse signal applied to its base from a luminance regulating IC 19.
Reference numeral 20 designates a diode serving to absorb a counter electromotive force of the choke coil 18.
The luminance regulating IC 19 regulates an output voltage level of the booster transformer 11 and thereby changes the luminance of the fluorescent lamp 21. To this end, the luminance regulating IC 19 outputs a pulse signal of a duty ratio depending on a set point for a regulating resistor 22 so that said pulse signal makes the switching transistor 17 conductive.
Reference numeral 23 designates a starter switch.
The output coil 11S of the booster transformer 11 is connected through a capacitor 24 to the fluorescent lamp 21.
A resistor 25, a diode 26 and a capacitor 27 form together a detector circuit for a load current and a voltage charged in the capacitor 27 is applied as a detection signal to the luminance regulating IC 19 through the regulating resistor 22.
With this converter of prior art as has been described above, upon closure of the starter switch 23, the luminance regulating IC 19 outputs the pulse signal of a duty ratio depending on the set point for the regulating resistor 22 and the switching transistor 17 responds to this pulse signal to be repeatedly turned ON and OFF.
Such operation of the switching transistor 17 causes the source current to be applied through this transistor 17 and the choke coil 18 to the center tap P of the booster transformer 11.
Thereupon a starting current is applied through the starting rheostat 16 to the bases of the respective transistors 13, 14 which are then switched from OFF-state to ON-state. It should be understood here that these two transistors 13, 14 are different from each other in transistor characteristic and circuit arrangement so that one of them becomes more positively conductive than the other and said one transistor is turned ON earlier than the other.
For example, assumed that the transistor 13 is turned ON earlier than the transistor 14, the source current will be supplied through the input coil 11P on one side with respect to the center tap P and the transistor 13.
When the input current flows through the input coil 11P, the inductive voltage generated across the feedback coil 11f is applied to the base of the transistor 13. This feedback effect accelerates the transistor 13 to be turned ON.
Transistor 13 is switched from ON to OFF at a time point depending on a magnetic saturation characteristic of the booster transformer 11 as well as on the transistor characteristics of said transistor 13 and then the transistor 14 is turned ON.
Now the source current is supplied through the input coil 11P on the other side with respect to the center tap P and the transistor 14, whereupon the inductive voltage generated across the feedback coil 11f is applied to the base of the transistor 14 to achieve the feedback effect.
Similarly to the case of the transistor 13, the transistor 14 is switched from ON to OFF at the time point depending on the magnetic saturation characteristic of the booster transformer 11 as well as on the transistor characteristics of said transistor 14 and then the transistor 13 is turned ON.
Alternate turning ON of the transistors 13, 14 in this manner causes the inverter to oscillate and thereby to generate high AC voltage across the output coil 11S.
The fluorescent lamp 21 is turned ON by the AC voltage output from the output coil 11S and the tube current (i.e., load current) is detected by a detector circuit comprising a resistor 25, a diode 26 and a capacitor 27.
Fluctuation in the tube current is thus detected and the detection signal is applied to the luminance regulating IC 19.
With the inverter of prior art as has been described above, the switching transistor 17 responds to the pulse signal output from the luminance regulating IC to be turned ON and OFF, so an amount of current applied to the input coil 11P of the booster transformer 11 depends on this pulse signal.
In other word, a level of AC voltage output from the booster transformer 11 depends on a duty ratio of said pulse signal.
Therefore, the duty ratio of the pulse signal output from the luminance regulating IC 19 may be adjusted in order to adjust a luminance of electric light provided by the fluorescent lamp 21 when the latter is turned ON.
However, the luminance regulating IC 19 employed by the above-mentioned inverter of prior art as means to regulate the luminance of electric light provided by the fluorescent lamp 21 inevitably complicates the circuit arrangement and increases the component cost.